As is well known the presence of vehicles is commonly detected by the influence they exert on the inductance of an a.c. energised loop laid in the road or other place where presence detection is required. The presence of a vehicle lowers the loop inductance. It is well established practice to monitor loop inductance changes by making the loop a frequency determining element of an oscillator circuit, for example by connecting it in circuit with capacitance to provide a resonant circuit for the oscillator so that the movement of a vehicle over the loop induces a frequency modulation (F.M.) of the oscillator. Changes in oscillator frequency are analyzed (for which many forms of detector circuitry are known) to indicate a vehicle presence. In certain circumstances problems can arise with these established techniques.
Two detector loops may be located in proximity and if the loops have similar inductances their operating frequencies may be sufficiently close to cause crosstalk due to coupling between the loops. This problem is compounded because as a vehicle moves over the loop the resultant F.M. sidebands can extend over a wide bandwidth.
In some countries the licencing authority will only allow operation within a specific frequency band making it more difficult to ensure that adjacent loop installations are sufficiently spaced in operating frequency. Additionally limits may be placed on frequency stability which are difficult to meet with free-running oscillators inherently subject to ambient changes.
There will be described hereinafter how the present invention can be put into practice to provide a detector loop installation in which the detector loop is operated at constant frequency yet a frequency output is obtained that varies with the loop inductance. Furthermore, it will be shown how a number of detector loops can operate under such constant frequency conditions, the operating frequency of each loop being independently selectable even when the loops have similar inductances.